Method of manufacturing zinc-coated electrode wire for electric discharge processors using hot dip galvanizing process

ABSTRACT

Disclosed is a method of manufacturing a zinc-coated electrode wire for electro discharge machining using a hot dip galvanizing process. The method includes firstly surface-forming a wire ( 1 ) in such a way that its terminal end is tapered while the wire is drawn through a first die at step  10 , pre-coating zinc around the firstly processed wire in a molten zinc bath ( 2 ) at step  20 , main-coating the pre-coated wire with zinc ( 3 ) using a sizing die before zinc pre-coated on the wire is hardened at step  30 , secondly surface-forming the main-coated wire in such a way that zinc coated around the wire has a uniform thickness using a second die having a smaller diameter than the wire at step  40 , heat-treating the secondly surface-processed wire at step  50 , and drawing the heat-treated wire using a third die ( 5 ) made of natural diamond at step  60.

TECHNICAL FIELD

The present invention relates to methods of manufacturing an electrodewire, which is useful as a cutting tool of a workpiece, using a hot dipgalvanizing process. More specifically, the present invention isdirected to a method of manufacturing a zinc-coated electrode wire forelectro discharge machining using a hot dip galvanizing process, bysubjecting a wire to the series of processes of firstly surface-forming,pre-coating, main-coating, secondly surface-forming, heat-treating anddrawing, which is advantageous in terms of uniformly coating zinc on thewire by the hot dip galvanizing process, and thus reducing manufacturingcost, thereby achieving economic benefits. In addition, environmentalcontamination by harmful gas and wastewater inevitably generatedaccording to conventional methods can be prevented by the presentinvention. Further, increase of both of thickness and adhesion of azinc-coated layer results in decreasing generation of waste powder uponpractical use of the wire, therefore improving the entire functions ofthe electrode wire.

BACKGROUND ART

In general, so-called electro discharge machining (hereinafter, referredto as “EDM”) is a well-known technique for the accurate machining of ametallic workpiece by arc heat upon electro discharge. As shown in FIG.9, when high voltage is applied between a workpiece and an electrode,large quantities of electrons flow close to the workpiece and arcdischarge takes place. Thereby, the workpiece can be machined to adesired shape.

With great advances in the relevant industries, such EDM has been widelyused to machine hardened materials that are difficult or impossible tomachine by other conventional methods.

The EDM using a wire type electrode material is known to be a wire-cutEDM. In particular, the wire-cut EDM is effective for machining of smallworkpieces having complex geometries, such as tools or dies. However,when the workpiece is continuously machined with the electrode wirewhile being applied with voltage, the end portion of the wire isexhausted and thus such a wire cannot be reused after electro discharge.

Further, heat generated by electro discharge may cause breakage of theelectrode wire. Thus, a brass wire alloyed with zinc (Zn) is mainly usedto decrease internal heat with vaporization of an electrode materialwith high vapor pressure upon electro discharge. Use of such a brasswire leads to improvement of electro discharge effect, but is restrictedby solid-solution limitations of conventional alloys. That is, themaximum content of zinc solid-soluble in a phase (FCC) constituting acoating layer amounts to about 39% at 456° C.

Although a copper wire is mainly used as an electrode wire for wireelectro discharge machining, it is low in tensile strength. Hence, hightension cannot be applied to the wire, and it is difficult to restrainvibration of the electrode wire upon electro discharge machining,causing rough machining of the workpiece and easy breakage of the wire.In addition, workability of electro discharge machining of copper per seis not good and a machining speed becomes slow.

Therefore, a brass electrode wire made of copper-zinc alloy is adaptableto increase the machining speed.

That is, as the content of zinc on the brass electrode wire increases,the machining speed becomes faster. This is because zinc functions tocause explosive electro discharge and efficiently remove molten portionsof the workpiece. Thereby, the workpiece is hardly attached withcontaminants.

As mentioned above, the higher the content of zinc, the higher theworkability of electro discharge machining. However, the wire is loweredin drawability.

In other words, when the coating layer contains more than 40% zincexceeding 40%, it is composed of a needle-like structure, and β phasehaving high hardness is formed therein. Thus, a drawing process isdifficult to perform.

Therefore, the electrode wire is coated with a brass layer having ashigh Zn content as possible within the limitation of drawability, so asto increase the machining speed and realize the accurate machining ofthe workpiece. In practical use, only a portion of several μm thick fromthe surface of the wire affects electro discharge machining. Even thoughthe same brass wire is used, workability of the electro dischargemachining varies according to the manufacturing method of such a brasswire.

The brass electrode wire on which zinc is coated is manufactured byvarious processes, for example, electroplating, hot dip galvanizing,plasma coating and thermal spraying, in which the plasma coating processand the thermal spraying process are disadvantageous in terms of highmanufacturing cost, and thus are scarcely used.

In addition, the hot dip galvanizing process is advantageous in light oflow manufacturing cost, and no generation of harmful gas and wastewater,thus causing no environmental problems, but suffers from drawbacks, suchas change of characteristics of the brass wire upon passing through abath consisting primarily of zinc molten at high temperature, andformation of the coating layer having non-uniform composition.

Although having the above-mentioned advantages, the hot dip galvanizingprocess is not practically used for preparation of the electrode wirefor electro discharge machining, and is limitedly used only for acorrosion resistant coating process.

The reason why the hot dip galvanizing process is not practically usedis that a surface temperature of the wire should be close to the meltingpoint of zinc so that the molten zinc is uniformly coated around thewire, because of surface tension of the molten zinc. Accordingly, thesurface temperature of the wire is increased to perform the coatingprocess, whereby zinc in the brass electrode wire is coated on the wirewhile being deposited.

As such, a diffusion reaction takes place, transferring a metal fromhigh concentration portion to a low concentration portion. Thereby, zincconcentration on the wire differs from that in the wire.

Further, since a diffused layer formed on the wire is composed of not auniform zinc composition but a non-uniform alloy composition, the hotdip galvanizing process is unsuitable for preparation of a zinc-coatedelectrode wire.

Moreover, the electrode wire is peeled during performing the electrodischarge machining. In such a case, properties of the wire when thedischarge machining is initiated vary from those of the wire when thedischarge machining is terminated, attributable to different propertiesbetween the surface and the inside of the coating layer. Thus, it isdifficult to uniformly perform the electro discharge machining by use ofthe electrode wire manufactured by the hot dip galvanizing process.

Therefore, the electroplating process, which is based on a principle ofcoating zinc ions in a coating solution to a surface of a wire byelectrical force, is mainly used to perform a zinc coating process onthe brass electrode wire. As a result, the zinc-coated layer havinguniform zinc composition is formed on the wire and thickness of such alayer is easily controllable.

However, the electroplating process is disadvantageous in light of highmanufacturing cost, generation of hazardous gases and wastewater, thusnegatively affecting the environment.

Thus, manufacturing methods of the zinc-coated brass electrode wirewithout the above problems are urgently required.

DISCLOSURE OF THE INVENTION

Therefore, it is an object of the present invention to alleviate theproblems encountered in the related art and to provide a method ofmanufacturing a zinc-coated electrode wire for electro dischargemachining by use of a hot dip galvanizing process, by subjecting a wireto the series of processes of firstly surface-forming, pre-coating,main-coating, secondly surface-forming, heat-treating and drawing. Theinventive method is advantageous in terms of formation of uniformlycoated zinc on the wire as in an electroplating process even through thehot dip galvanizing process is used, resulting in low manufacturingcost, thereby achieving economic benefits. In addition, environmentalcontamination problems by harmful gas and wastewater inevitablygenerated according to conventional methods can be prevented. Further,increase of thickness and adhesion of a zinc-coated layer leads todecreased generation of waste powder, thus improving the entirefunctions of the electrode wire.

In order to achieve the above object, the present invention provides amethod of manufacturing a zinc-coated electrode wire for electrodischarge machining using a hot dip galvanizing process, the methodcomprising the following steps of firstly surface-forming a wire so thatits terminal end is tapered while the wire is drawn through a first die;pre-coating the firstly surface-processed wire with zinc by passing thewire through a molten zinc bath at a relatively slow speed; main-coatingthe pre-coated wire with zinc, wherein the wire is immersed in themolten zinc bath for a predetermined time to maintain the temperature ofzinc pre-coated on the wire at a predetermined level, and is removedfrom the molten zinc bath and then passed through a sizing die preheatedto 400° C. before zinc coated on the wire is hardened, so that zinccoated on the wire has a predetermined thickness; secondlysurface-forming the main-coated wire by passing the wire through aheated pipe at a constant speed to raise a surface temperature of thewire to a predetermined level, and then passing the wire through asecond die having a diameter of 5-10 μm smaller than that of the wire sothat zinc is coated around the wire at a uniform thickness;homogeneously heat-treating the secondly surface-processed wire in aclosed space by hot air circulating therein; and drawing thehomogeneously heat-treated wire with a drawing ratio of 4-80 or higherby passing the wire through a third die made of natural diamond andhaving an inlet portion of 5 μm across, a middle portion of 3 μm acrossand an outlet portion of 1˜3 μm across to make the surface of the wiresmooth, provided that the homogeneously heat-treated wire has asectional area of 0.3˜3 mm².

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects, features and advantages of the present invention willbe more clearly understood from the following detailed description takenin conjunction with the accompanying drawings, in which:

FIG. 1 is a view illustrating overall processes of a method ofmanufacturing a zinc-coated electrode wire for electro dischargemachining using a hot dip galvanizing process, according to the presentinvention;

FIG. 2 is a view illustrating a coating process by use of a molten bathin the method of manufacturing a zinc-coated electrode wire for electrodischarge machining using a hot dip galvanizing process, according tothe present invention;

FIG. 3 is a photograph illustrating the wire subjected to a firstlysurface-forming process in the method of manufacturing a zinc-coatedelectrode wire for electro discharge machining using a hot dipgalvanizing process, according to the present invention;

FIG. 4 is a photograph illustrating the wire subjected to a pre-coatingprocess in the method of manufacturing a zinc-coated electrode wire forelectro discharge machining using a hot dip galvanizing process,according to the present invention;

FIG. 5 is a photograph illustrating the wire subjected to a main-coatingprocess in the method of manufacturing a zinc-coated electrode wire forelectro discharge machining using a hot dip galvanizing process,according to the present invention;

FIG. 6 is a photograph illustrating the wire subjected to a secondlysurface-forming process in the method of manufacturing a zinc-coatedelectrode wire for electro discharge machining using a hot dipgalvanizing process, according to the present invention;

FIG. 7 is a photograph illustrating the wire subjected to ahomogeneously heat-treating process in the method of manufacturing azinc-coated electrode wire for electro discharge machining using a hotdip galvanizing process, according to the present invention;

FIG. 8 is a view illustrating a main portion of a die used for a drawingprocess in the method of manufacturing a zinc-coated electrode wire forelectro discharge machining using a hot dip galvanizing process,according to the present invention; and

FIG. 9 is a view illustrating use of the zinc-coated electrode wire forelectro discharge machining using a hot dip galvanizing process,according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

With reference to FIG. 1, there is illustrated a manufacturing method ofa zinc-coated electrode wire for electro discharge machining using a hotdip galvanizing process, according to the present invention. Inaddition, FIG. 2 schematically illustrates a coating process by use of amolten bath, according to the manufacturing method of the zinc-coatedelectrode wire for electro discharge machining using a hot dipgalvanizing process.

In the present invention, a specific description for the relatedtechniques or structures is considered to be unnecessary and thus isomitted.

Further, before the manufacturing method of the present invention isdisclosed, it should be understood that the terminology used therein isfor the purpose of describing particular embodiments only and is notintended to be limiting.

As shown in FIG. 1, a wire 1 is firstly surface-processed in such a waythat its terminal end is tapered while the wire 1 is drawn through afirst die, at step 10. The firstly surface-processed wire 1 ispre-coated with zinc 3 at step 20, by passing through a molten zinc bath2 heated to 440-500° C. at a relatively slow speed of 30-40 m/min sothat the wire 1 is immersed in the molten zinc bath 2 for 1˜2 sec. Assuch, the firstly surface-processed wire 1 has a sectional area of 0.3˜3mm².

Then, the pre-coated wire 1 is main-coated with zinc 3 at step 30, bypassing through the molten zinc bath 2 heated to 430˜480° C. at 50-70m/min so that the wire 1 is immersed in the molten zinc bath 2 for 1-2sec. In such a case, zinc 3 pre-coated around the wire 1 is at atemperature of 410±10° C. Immediately after the wire 1 on which zinc 3is coated is removed from the molten zinc bath 2, it passes through asizing die preheated to 400° C. before a zinc-coated layer formed on thewire 1 is hardened. Thereby, the zinc-coated layer is formed on the wireat a predetermined thickness.

Thereafter, the main-coated wire 1 is secondly surface-processed at step40, by passing through a 4˜6 m long pipe heated to 400° C. at 30˜50m/min so that a surface temperature of the wire 1 reaches 250-350° C.,and by passing through a second die having a diameter of about 5-10 μmsmaller than that of the wire 1. Thereby, zinc 3 coated around the wire1 has a uniform thickness.

The secondly surface-processed wire 1 is homogeneously heat-treated byhot air of 120˜180° C. circulating at 10˜20 m/sec in a closed space, atstep 50. Then, the heat-treated wire 1 is drawn at step 60, by passingthrough a third die 5 made of natural diamond 4 and having an inletportion of 5 μm across, a middle portion of 3 μm across and an outletportion of 1-3 μm across, provided that the homogeneously heat-treatedwire 1 has a sectional area of 0.3˜3 mm². Consequently, the electrodewire 1 having smooth surface and thin thickness can be obtained.

Turning now to FIG. 3, there is shown a photograph of the firstlysurface-processed wire according to the method of manufacturing thezinc-coated electrode wire for electro discharge machining using the hotdip galvanizing process. In addition, FIG. 4 illustrates a photograph ofthe pre-coated wire according to the method of manufacturing thezinc-coated electrode wire for electro discharge machining using the hotdip galvanizing process. Further, FIGS. 5 and 6 illustrate photographsof the main-coated wire and the secondly surface-processed wire,respectively, according to the method of manufacturing the zinc-coatedelectrode wire for electro discharge machining using the hot dipgalvanizing process. Furthermore, FIG. 7 illustrates a photograph of thehomogeneously heat-treated wire according to the method of manufacturingthe zinc-coated electrode wire for electro discharge machining using thehot dip galvanizing process. Also, FIG. 8 illustrates the third die usedfor the drawing process according to the method of manufacturing thezinc-coated electrode wire for electro discharge machining using the hotdip galvanizing process.

As mentioned above, the zinc-coated electrode wire is manufactured bysubjecting the wire to a series of the processes of firstlysurface-forming, pre-coating, main-coating, secondly surface-forming,heat-treating and drawing. Thereby, the electrode wire for electrodischarge machining can be uniformly coated with zinc at an outersurface thereof even by the hot dip galvanizing process, as inelectroplating process.

Specifically, as for the firstly surface-forming process at step 10, thewire 1 is surface-processed in such a way that its terminal end istapered as shown in FIG. 3, while the wire 1 is drawn through the firstdie. Thereby, a diffusion reaction by high temperatures takes place onlyat the tapered end of the wire 1 upon passing through the molten zincbath 2.

Hence, diffusion phenomena of a brass electrode wire at hightemperatures are minimized, and thus volume of the electrode wire ischanged minimally.

Subsequently, as for the pre-coating process at step 20, the firstlyprocessed wire 1 passes through the molten zinc bath 2 heated to440˜500° C. at a relatively slow speed of 30˜40 m/min when it has asectional area of 0.3˜3 mm², so that the wire 1 is immersed in themolten zinc bath 2 for 1-2 sec. Thereby, zinc 3 is coated around thewire 1, as shown in FIG. 4.

As such, if the immersion time of the wire is shorter than 1˜2 sec, thediffusion reaction of the wire does not occur and thus the coatingprocess cannot be effectively carried out. Meanwhile, if the immersiontime is longer than 1˜2 sec, the diffusion reaction occurs excessively,thus forming a diffused layer with extremely wide area and thick layerthickness, causing a non-uniformly coated wire.

As for the main-coating process at step 30, the wire 1 pre-coated atstep 20 passes through the molten zinc bath 2 heated to 430˜480° C. at50˜70 m/min so that the wire 1 is immersed in the molten zinc bath 2 for1˜2 sec. Thereby, the temperature of zinc 3 coated around the wire 1reaches 410±10° C. Immediately after such a wire 1 is removed from themolten zinc bath 2, it passes through the sizing die preheated to 400°C. before the zinc 3 coated on the wire is hardened, whereby zinc 3 iscoated around the wire 1 at a predetermined thickness, as shown in FIG.5.

Next, as for the secondly surface-forming process at step 40, themain-coated wire 1 passes through a 4-6 m long pipe heated to 400° C. at30˜50 m/min to reach the surface temperature of the wire 1 250˜350° C.,and then passes through the second die having a diameter of about 5˜10μm smaller than that of the wire 1. Thereby, zinc 3 is uniformly coatedon the wire 1, as shown in FIG. 6.

As for the homogeneously heat-treating process at step 50, the secondlysurface-processed wire 1 is placed into a closed space, and ishomogeneously heated by the hot air of 120˜180° C. circulating at 10-20m/sec therein, as shown in FIG. 7.

Such a heat-treating process is performed with the intention ofimproving low adhesion of zinc 3 coated on the surface-formed wire 1 aswell as increasing stability of zinc particles per se.

Through the heat-treating process, a diffused layer about 1 μm thick isformed at a contact surface between the wire 1 and zinc 3 coated on thewire 1, whereby the zinc-coated layer can be firmly bonded to the wire1. Further, zinc 3 on the electrode wire 1 is converted to zinc oxide,and causes the wire to have high rigidity. Accordingly, the electrodewire of the present invention has no conventional problems, such asgeneration of waste powder and breakage of the wire as in the drawingprocess of conventional electrode wires coated with only zinc.

As for the drawing process at step 60, the heat-treated wire 1 passesthrough the third die 5 made of natural diamond 4 and having an inletportion of 5 μm across, a middle portion of 3 μm across and an outletportion of 1˜3 μm across, as shown in FIG. 8, to manufacture a smoothand thin electrode wire, provided that the homogeneously heat-treatedwire 1 has a sectional area of 0.3˜3 mm².

In particular, upon performing the drawing process of the wire with adrawing ratio of 4-80 or higher, such a diamond die 5 having an inletportion of 5 μm, a middle portion of 3 μm and an outlet portion of 1˜3μm is favorable so as to preserve the bonded state of zinc coated on thewire, which is unstable due to very different physical properties of thewire and zinc on the wire.

After the drawing process, final heat treatment and productmanufacturing processes are the same as conventional methods anddescription thereof is omitted.

INDUSTRIAL APPLICABILITY

As described above, the present invention provides a method ofmanufacturing a zinc-coated electrode wire for electro dischargemachining using a hot dip galvanizing process, and the method ischaracterized by subjecting a wire to firstly surface-forming,pre-coating, main-coating, secondly surface processing, uniformlyheat-treating and drawing treatments. Even though the hot dipgalvanizing process is used, the wire can be uniformly coated with zincat an outer surface thereof as in an electroplating process. Thereby,the inventive method is advantageous in terms of decreased manufacturingcost, thus generating economic benefits.

In addition, the inventive method is very effective for prevention ofenvironmental contamination due to noxious gases and wastewatergenerated by conventional manufacturing methods.

Further, the electrode wire manufactured by the inventive method isimproved in adhesion between the wire and zinc coated thickly thereon,and thus decreases generation of waste powder upon practical usethereof. Consequently, the electrode wire can be enhanced in overallfunctions thereof.

The present invention has been described in an illustrative manner, andit should be understood that the terminology used is intended to be inthe nature of description rather than of limitation. Many modificationsand variations of the present invention are possible in light of theabove teachings. Therefore, it should be understood that within thescope of the appended claims, the invention may be practiced otherwisethan as specifically described.

1. A method of manufacturing a zinc-coated electrode wire for electrodischarge machining using a hot dip galvanizing process, the methodcomprising the following steps of: firstly surface-forming a wire sothat a terminal end of the wire is tapered while the wire is drawnthrough a first die; pre-coating the firstly surface-processed wire withzinc by passing the wire through a molten zinc bath at a relatively slowspeed; main-coating the pre-coated wire with zinc, wherein thepre-coated wire is immersed in the molten zinc bath for a predeterminedtime to maintain the temperature of zinc pre-coated on the wire at apredetermined level, and is removed from the molten zinc bath and thenpassed through a sizing die preheated to 400° C. before zinc coated onthe wire is hardened, so that zinc coated on the wire has apredetermined thickness; secondly surface-forming the main-coated wireby passing the wire through a heated pipe at a constant speed to raise asurface temperature of the wire to a predetermined level, and thenpassing the wire through a second die having a diameter of 5˜10 μmsmaller than that of the wire so that zinc is coated around the wire ata uniform thickness; homogeneously heat-treating the secondlysurface-processed wire in a closed space by hot air circulating therein;and drawing the homogeneously heat-treated wire with a drawing ratio of4˜80 or higher by passing the wire through a third die made of naturaldiamond and having an inlet portion of 5 μm across, a middle portion of3 μm across and an outlet portion of 1˜3 μm across to make the surfaceof the wire smooth, provided that the homogeneously heat-treated wirehas a sectional area of 0.3˜3 mm².
 2. The method as defined in claim 1,wherein, at the pre-coating step, the firstly surface-processed wirepasses through the molten zinc bath heated to 440˜500° C. at arelatively slow speed of 30˜40 m/min when the wire has a sectional areaof 0.3˜3 mm², so that the wire is immersed in the molten zinc bath for1˜2 sec.
 3. The method as defined in claim 1, wherein, at themain-coating step, the pre-coated wire passes through the molten zincbath heated to 430˜480° C. at 50˜70 m/min so that the wire is immersedin the molten zinc bath for 1˜2 sec.
 4. The method as defined in claim1, wherein, at the secondly surface-forming step, the main-coated wirepasses through the pipe heated to 400° C. at 30˜50 m/min, so that thesurface temperature of the wire reaches 250 350° C.
 5. The method asdefined in claim 1, wherein, at the homogeneously heat-treating step,the secondly surface-processed wire is heated by the hot air of 120˜180°C. circulating at 10˜20 m/sec in the closed space.